Technological advancements of the last several years have enabled satellite based systems to offer voice and data services to mobile terminals on a global basis. These systems can also provide voice and data services to fixed installation terminals, thereby enabling basic telephony services in areas lacking a terrestrial telecommunications infrastructure. Primary objectives of these personal communication satellite services (PCSS) are to offer the services at low prices and to provide a high level of service quality.
In recent years satellite based systems have been proposed which offer direct communication between mobile or fixed terminals and satellites arranged at low and medium altitudes. The communications links include traffic channels over which voice information or data are transmitted. Proposed satellite based telecommunications systems utilize earth stations to interconnect through gateways with conventional terrestrial phone networks. The earth stations may also route communications between satellites and terminals. The earth stations may further provide control over signaling, transmission timing and transmission frequency of the terminals as necessary to establish and maintain calls directed to or initiated by terminals.
Examples of recently proposed satellite based systems include the Globalstar(trademark) system proposed by Globalstar(trademark) Telecommunications Limited, the Iridium system proposed by Motorola Inc., and the Odyssey system proposed by TRW and Teleglobe.
The earth stations, satellites and mobile terminals communicate via a predefined waveform format. The waveform format supports forward communications links from earth stations through satellites to terminals. The waveform format also supports return communications links from the terminal through the satellite to the earth station. The design of the communications waveform format for a system plays a significant role in meeting the system""s objectives such as enhancing bandwidth efficiency, enhancing satellite power usage efficiency, providing rapid terminal acquisition, providing robust communications links and maintaining user privacy. Further objectives include maximizing the number of simultaneous terminals that a system may support while minimizing the capital cost of the system. The number of terminals supported by a single satellite depends in part on the available bandwidth for communications between the satellite and terminals and between the satellite and earth stations. The number of.terminals also depends upon the power required by each terminal, the satellites RF transmission capability, physical environment factors (e.g., necessary link margins), regulatory constraints (e.g., terminal radiated power constraints, satellite power flux density constraints, out-of-band emissions constraints, etc.) and the like.
Communications waveform formats have been proposed in the past, such as the Telecommunication Industry Association/Electronics Industries Association Interim Standard 95 (IS-95) proposed by QUALCOMM, Inc., of San Diego, California, with some cooperative effort from ATandT, Motorola and others. IS-95 incorporates CDMA modulation techniques disclosed in U.S. Pat. No. 5,103,459. IS-95 describes a code division multiple access (CDMA) waveform format, in which multiple terminals communicate in a common bandwidth or subband. In this common subband, terminals are distinguished from one another by a code uniquely assigned to each terminal. The CDMA code may also be referred to as a codeword or xe2x80x9cchip codexe2x80x9d. The chip code represents a pseudo-noise (PN) spreading code which xe2x80x9cspreadsxe2x80x9d the signal over the available bandwidth and allows more terminals to communicate over the same frequency range. The chip code is combined or modulated with information bits which define a voice or data signal. The combined data stream of voice or data and the chip code is divided into frames and transmitted over a traffic channel. The chip code is transmitted at a rate (the chip rate) much faster than the information bit rate.
In the IS-95 waveform format, a single CDMA subband is 1.23 MHz wide and will support a theoretical maximum of 63 terminals or subscribers with unique CDMA codes. In practice, the transmissions to and from the terminals interfere with one another and unduly degrade the quality of each communications link if more than approximately 30 terminals share a subband for satellite application. In terrestrial application as few as 12 terminals may be able to share a subband. This type of interference is referred to as xe2x80x9cmultiple access interferencexe2x80x9d. The IS-95 waveform format and CDMA generally are explained in more detail in chapter 13 of a book entitled xe2x80x9cAn Introduction to GSMxe2x80x9d, by Siegmund H. Reidl, Matthias K. Weber and Malcolm W. Oliphant, published by Artech House, Inc., of Norwood, Mass., 1995. Chapter 13 of the above-referenced book is expressly incorporated herein by reference.
However, CDMA systems thus far proposed have met with limited success. By way of example, the IS-95 waveform affords an asynchronous return link (i.e., from the terminal to the earth station) which unduly limits the number of terminals that may simultaneously communicate over a limited bandwidth.
In xe2x80x9casynchronousxe2x80x9d CDMA terminals transmit communications to an earth station independent in time from one another. This results in far larger multiple access interference than may result with orthogonal CDMA.
Further, the IS-95 waveform uses a combination of xe2x80x9copen loopxe2x80x9d and xe2x80x9cclosed loopxe2x80x9d methods for controlling signal power transmitted by the terminal in the return link. The terminal may adjust return link transmission power in part based on the power received on the forward link (i.e., open loop control). However, this open loop power control routine is inaccurate since power fluctuations of the signals on the forward and return links are not necessarily correlated to one another.
Moreover, the IS-95 waveform makes inefficient use of the available bandwidth by requiring every frame in the traffic channel to include xe2x80x9ctail bitsxe2x80x9d to convert the convolutional code into a block code. These tail bits reduce the transmission rate for voice or data.
A need remains within the industry for an improved communications waveform for use in satellite based cellular telecommunications.
It is an object of the present invention to provide a communications waveform format with enhanced bandwidth efficiency.
It is a corollary object of the present invention to provide a communications waveform format which utilizes orthogonal CDMA codes in the forward and return links to minimize multiple access interference between terminals, thereby increasing the number of terminals which may be supported per unit of allocated bandwidth.
It is another corollary object of the present invention to provide an orthogonal CDMA communications waveform format which uses a set of quadratic residue orthogonal CDMA codes, each of which allow a highly precise match between the information rate and the desired chip rate.
It is a further object of the present invention to provide a communications waveform format that provides continuous updates to return link transmitted power, frequency and timing, thereby enabling an orthogonal synchronous return link.
It is a further object of the present invention to provide a communications waveform format having a return link sync field which permits terminals to be independently tracked in a dense CDMA environment.
It is a further object of the present invention to provide constant envelope return link modulation, which provides low levels of unwanted power emissions from terminals while using inexpensive saturating amplifiers.
It is yet another object of the present invention to provide a communications waveform format having signaling transition frames used to signal a change of traffic channels between active and inactive states to avoid the need to transmit the number of tail bits with each frame.
It is yet a further object of the present invention to provide a communications waveform format in which the transmission power of the return link is varied during the transmission of frames to reduce self interference on the return link.
Another object of the present invention is to provide a communications waveform format with a reduced return link chip rate, while maintaining the same information rate on the forward and return links, thereby allowing a balanced use of forward and return links even though the return link operates with a smaller bandwidth allocation.
Another object of the present invention is to provide a communications waveform format which uses punctured convolutional codes to match information rates to the symbol rate, thereby enhancing satellite power usage efficiency.
It is yet a further object of the present invention to provide a communications waveform format which utilizes variable transmitted power on the forward link which may vary from symbol to symbol to further enhance satellite power usage efficiency.
It is even a further object of the present invention to provide a communications waveform format in which the traffic channel is turned inactive and the information bit rate is reduced to zero during periods in which no data or voice is transmitted.
It is a further object of the present to provide a communications waveform format which utilizes interleaving to mitigate the impact of an imperfect carrier phase reference.
It is a further object of the present invention to provide a communications waveform format having an isolation code which is the same for each frame, simplifying acquisition for terminals.
It is a further object of the present invention to provide a communications waveform format having an asynchronous return access channel to respond to pages and to initiate calls which provides small interference to orthogonal CDMA usage without requiring inefficient use of the available spectrum.
A communications waveform format is provided for a satellite-based telecommunications system. The waveform format includes forward and return link waveforms for the feeder links between the satellite and earth stations. The waveform format further includes forward and return link waveforms for the terminal links between the satellite and terminals (mobile or fixed). The satellites operate as xe2x80x9cbent pipesxe2x80x9d and perform frequency translation and signal filtering between feeder and terminal links, without effecting significant changes to the detailed waveform structure.
The forward and return links use orthogonal direct-sequence code division multiple access (ODS-CDMA) to minimize interference between users. The forward feeder link baseband spectrum may be divided into feeder channels, the transmitted frequencies and spacing of which are adjustable to compensate for Doppler. effects, thereby maintaining synchronization between earth stations sharing a satellite. Each feeder channel is translated in frequency by the satellite and routed to a specific beam to be output as a forward terminal link. The spectrum of the forward terminal link may be divided into multiple subbands (e.g., 38), each of which may support multiple (e.g., 80) ODS-CDMA circuits or channels for individual mobile or fixed terminals. At least one feeder channel may be designated to carry only pilot tones as reference signals for the satellite. Pilot tones are used by satellites to determine proper power levels of incoming signals and enable users to be combined on common feeder links with the correct relative power.
The forward feeder link and forward terminal link support traffic channels (TCH), associated signaling channels (ASC), broadcast control channels (BCC), forward signaling channels (FSC) having paging slots (PAS) and channel allocation slots (CAS), call establishment channels (CEC), and loop signaling channels (LSC). The TCH is a dedicated channel assigned to a given fixed or mobile terminal. The ASC conveys information between the earth station and a terminal on an as needed basis for handovers or call terminations, with the ASC replacing the TCH. The BCC broadcasts information required by a terminal to determine the proper beam to monitor for pages (when a terminal is called) and to use for requesting access to the system (when a terminal initiates a call).
The FSC is a common signaling channel that is monitored by multiple users for information such as pages (PAS) addressed to particular users and channel allocation slots (CAS). The PAS notifies a particular terminal of an incoming call. The CAS informs a terminal of a CEC that the terminal should use for call setup, as well as information required to bring the terminal into synchronization. The CEC exchanges call setup information between the terminal and earth station. The LSC contains control loop information for characteristics of the waveform, such as frequency, chip timing and transmission power.
The return terminal link spectrum may be divided into multiple subbands (e.g., 58), from which a subset of subbands (e.g., 38) may be used. The terminals correct frequency and timing of the return link subbands based initially on correction information transmitted over the CAS with the LSC used to maintain synchronization. As in the forward link, each subband may support multiple (e.g., 80) channels for individual terminals.
The return feeder link spectrum may be divided into multiple feeder channels (e.g., 122), one of which may be devoted to satellite telemetry. The terminals adjust the carrier frequencies as commanded via the LSC to compensate for the Doppler effect in order to maintain synchronization between satellites sharing the satellite.
The return link supports traffic channels (TCH), return associated signaling channels (ASC), a measurement reporting channel (MRC), a return loop signaling channel (LSC) a return call establishment channel (CEC), and a return access channel (RAC). The TCH, ASC and CEC perform the same functions as in the forward link. The MRC transmits environment data to earth stations regarding signal quality in potential handover candidate beams. The LSC reports the received signal quality measured on the forward link traffic channel. The RAC carries nonsynchronous spread spectrum pseudo-noise (PN) signal access bursts which are used by terminals to initially access the communications system.